The manufacture of the wafers for integrated circuits involves the formation of a series of layers or films upon a silicon wafer. In the typical manufacturing process, a wafer undergoes a photo resist step followed by photo lithography. The wafer is then etched, stripped and then subjected to a diffusion step. The planarization process is then used to planarize the surface of the wafer before the wafer is subjected to repeated iterations of these steps which build multiple layers on the wafer. CMP has been described as "wet sanding" the surface of the wafer and, the object of the planarization process is to achieve a highly uniform planar surface on the wafer without excessive removal or an insufficient removal of the film.
It is desirable to have each layer on the wafer extremely flat or planar before the creation of each successive layer mainly to improve the lithography and enable better control of the shrinking dimension. Before the incorporation of CMP processes, the topography of the wafer is irregular and, as a result, a lithographic image formed on the surface of the wafer may be out of focus. The lack of focus will interfere with subsequent etching steps which results in a larger uncertainty of process control and forces a larger ground rule.
The typical apparatus used in the CMP process includes a carrier which holds a wafer. This assembly is rotated and pushed down with the surface of the wafer which is to be polished oriented toward an abrasive pad. The abrasive pad is mounted on a platen which rotates on a different axis than the carrier. Typically, a water based slurry containing uniformly distributed and suspended small abrasive particles is introduced to the top surface of the rotating pad which contributes mechanical and/or chemical elements to the process. When the CMP process is directed to the removal of oxide layer on a wafer, the slurry is typically made of a water-based solution containing fumed silica with its pH between 10 and 11, adjusted by KOH. In contrast, when the slurry is intended for the polishing of a metallic layer, the slurry will typically contain oxidizers and have a low pH (0.5 to 4). New slurry is continuously introduced to the process as old slurry is removed. The old slurry will contain the abraded particles of both the pad and the wafer and end products of the chemical reaction. Following the CMP process the wafer is buffed to remove any slurry and cleaned to provide a smooth final finish to the wafer.
The CMP process is used for the planarization of both dielectric and metallic layers. The CMP process when applied to surfaces comprising metallic layers involves a chemical reaction at the surface of the wafer which leaves the surface more susceptible to mechanical abrasion by the particles suspended in the slurry. However, mechanical abrasive action dominates when the CMP process is directed toward oxide polishing. Notwithstanding, it has been found that the removal of oxide layers containing silicon nitride involves significant mechanical and chemical activity. Mechanical abrasive action of a silicon nitride layer creates a large surface area due to the numerous silicon nitride particles created in the process and therefore the chemical reaction yield is enhanced. At low pH the main product is NH.sub.4 + and at high pH it exits mainly as NH.sub.3.
There are a wide number of variables which can influence the rate and uniformity of the planarization operation. Mechanical variables involved in the process include the rotational speeds of the platen and carrier, the back pressure applied to the pad, the profile of the pad and carrier and the downward force applied to the platen. Further, the various components used in the process contribute to yet additional variables in the operation. Typically, the polishing pad on the platen is made of polyurethane and the surface is much rougher than the typical wafer being polished. Variation in the polishing pads or other consumables used in the process can significantly alter the rate of removal of layers on a wafer. In addition to the mechanical control parameters, the characteristics of the slurry add a number of additional variables, including the particle size distribution, temperature, pH and its rate of introduction. The CMP process itself is dynamic which further contributes to difficulties in precisely controlling the process and achieving repeatable results.
The total polishing time will depend on the initial film thickness and the film removal rate. The removal rate is dependent on the various process parameters identified above. To improve throughput, material removal rate can be maximized. Removal rate and removal rate uniformity across a wafer are also strong functions of downforce, platen speed, pad structure and slurry chemistry. Unfortunately, optimizing parameters for maximum removal rate is in conflict with optimized planarization. Thus, processes vary by application and the priority of the desired responses. During the development of a new device, considerable time is expended selecting the optimal mechanical parameters for the CMP process including the polishing pad features, the slurry characteristics, and the speeds and force between the platen and carrier. In the past this process has heavily relied upon experimental data relating to uniformity and film removal rate.
The output of the CMP process is planarized/polished wafers. Typical output characteristics being measured and monitored are the amount of material removed and its removal rate, within-wafer non-uniformity of material removed, wafer-to-wafer non-uniformity of the removal rate, the degree of planarization, as well as surface defects and contamination. The difference between pre- and post-CMP measurements provides information on the change caused by the process, such as the amount removed and the removal non-uniformity. Within-wafer non-uniformity is commonly measured as a standard deviation of the thickness removal measurements expressed as a percentage of the average thickness removed. Wafer-to-wafer non-uniformity is a standard deviation of removal rate for a number of wafers expressed as a percent of the average removal rate. Higher down-force reduces polish time and hence, increases the throughput but may adversely affect planarization and possibly non-uniformity.
In view of the numerous variables which effect the CMP process, optimizing the control of the process without an effective endpoint detection system presents difficulties. Merely attempting to precisely repeat operating conditions based upon historical results does not achieve reliable or satisfactory results. Removal of the wafer from the polishing table for periodic inspection and analysis significantly reduces the throughput and can introduce further variables into the process. In response to the need to monitor and control the progress of the CMP process, and more particularly, to accurately identify the endpoint of the process, a number of in situ techniques have been developed to monitor the rate of removal. For layers having metal components, a number of techniques have been used which either measure motor current changes or the frequency shift of a resonant circuit due to inductance change. With particular respect to the measurement of dielectric layers where the aforesaid techniques are ineffective, efforts have included (1) measuring the changes in frictional forces between the pad and the wafer by more sensitive means, (2) providing a window through the polishing pad or from the back of the wafer and, using an infrared light source, and then measuring changes in optical properties of the film being removed, and (3) sensing the acoustical changes in the process exhibited by different layers.
At present shallow trench isolation CMP requires the complete exposure of silicon nitride at the interface of the top two films without excessive removal of silicon nitride. BPSG CMP usually stops somewhere at the interface when the silicon nitride layer is partially exposed.
There is a continuing need for improved methods and devices for the in situ monitoring of the CMP process. In particular, for shallow trench isolation CMP process optimization, with any adjustment of any of the parameters, the goal is to stop the wafer polishing process at a time when all the nitride area has just been cleared. The identification of this point is very difficult because each change of the multiple factors which effect the CMP process will change the optimal stop time. Preferably detection of changes in process conditions should have a high degree of sensitivity and a rapid response time, preferably less than 1 or 2 seconds.